Chap. XIII.] GONADS OF ARTHROPODA. 499 



epithelium is found, are given off; in My sis the 

 caeca are fewer and more distinct, and in Oniscus 

 there are a few very long cseca. 



In lulus there are two testicular tubes united by a 

 number of median branches, and provided at their 

 sides with about as many rounded testicular follicles. 

 In insects the testes are ordinarily found to consist of 

 a large number of separate tubes, but the form of the 

 compact mass varies very considerably, and no 

 observations seem to have been made on these parts 

 since the discovery of the character of the true testi- 

 cular organs of the cockroach. 



While the spermatozoa of all Crustacea, with the 

 exception of the parasitic Cirripedia, have no power of 

 independent movement, those of the Insecta are wavy, 

 and one end is often rigid, those of Myriopods may be 

 rigid or motile, and those of the Arachnida, with 

 which Limulus must be classified, are often actively 

 motile. 



The hermaphroditic arrangements which obtain in 

 the Cirripedia are to be explained by their fixed 

 mode of life, while the imperative necessity of 

 avoiding the dangers of repeated self -fertilisation has 

 in some cases been yielded to in the production of 

 minute (^ inch) and degraded males (comple- 

 mental males) (Darwin), which, as in the case of 

 the Gephyrean Bonellia, attach themselves to the body 

 of the hermaphrodite, or simply female Cirriped. 



In other epizoic Crustacean parasites (Achtheres 

 percarum and other Siphonostomata) the male is con- 

 stantly smaller than, and is generally found attached 

 to, the female ; here, too, as in the case of the Rotifera, 

 the number of males is much smaller than that of the 

 females, and adult forms are often developed which 

 arise from non-fertilised ova. 



In some Isopoda (e.g. Cymothoa) a parasitic habit 

 likewise obtains, and there is a curious mixture of 



